It's interesting and warrants further study. I have a few thoughts about the phenomenon. When talking to yourself, whether alone or with other people in earshot, you're only talking to yourself; only you need to understand what you're saying. You might speak fluently or you might mumble and it doesn't matter because there is no intended audience but yourself. When talking to someone else, it's entirely different - it becomes a social act. Not only do you want to be understood by another person, other societal pressures come into play. Factors include other people's perceptions of you, your own perceptions of how you appear, your own perceptions of other people's perceptions. If you already have a nervous disposition this can escalate to social anxiety and create a feedback loop which intensifies anxiety creating more difficulty with speech. Even if you are not nervous or anxious, the social element still interacts with the neurological condition, which manifests as disrupted speech, to some degree. Attempting to talk to someone without the expectation of being heard is a contradiction in terms. Talking to someone necessitates being heard. Otherwise you're just talking AT someone, and I'm sure we all know people who do that, who don't care whether someone is listening (I've done it myself on occasion). But that's not effective communication. I find that if I'm clear on what I'm saying and the other person is listening, speech is usually ok (this is having spent years dealing with psychological issues which contributed to speech difficulties). If I'm not clear, or they're not listening, or there are distractions in the environment or in the mind, I'll have some difficulty. If we think more in terms of communicating effectively, which is a social interactive phenomenon, rather than focusing on the mechanics of speech, then we can remove some of the barriers to fluent speech. That's not to say we shouldn't look at the mechanics of speech at all. I'm all for practicing techniques, improving diction, tongue-twisters, etc. These things can be done alone or with a coach, not in the middle of a conversation. I don't practice a guitar piece on stage. I practice alone or with the band so when on stage I can just do it. In a social situation, there's so much going on, as described above, that it can be difficult to focus on mechanics, so get those worked out beforehand.